Stress or magnetic field sensor with spatially varying bias

ABSTRACT

A stress or magnetic field sensor comprising a generally elongate magnetically soft amorphous or nanocrystalline electrically resistive element and biasing means for applying to the element a bias magnetic field of which the component directed along the length of the sensor has an amplitude variation pattern along the element. A periodically varying pattern has the effect of reducing the sensitivity of a stress sensor to external ambient fields (FIG.  3  shows that with a sawtooth bias field the sensitive portions a of a sensor move to positions b in the presence of an ambient field, but their number remains the same). A ramped bias field enables the position of the sensitive region of the sensor to be controlled, for measuring local stress, or for mapping an external magnetic field. Control of the regions where the sensor is active may include selective conductive coating of portions of its length. Potential uses of the stress sensor include a pressure sensor, embedment in moving parts (using rf communication) such as vehicle tyres, aircraft wings or machine parts, and in structures such as bridges where stray magnetic fields are a problem.

This application is a 371 of PCT/GB02/01298 dated Mar. 20, 2002.

The present invention relates to the measurement of stress usingmagnetic stress impedance sensors.

Of the many methods for measuring stress that are known in the priorart, few or none fulfil all the requirements of low cost, highrobustness and high sensitivity which are the ideal for manyapplications. Additional constraints may arise when it is also requiredthat the stress to be measured is in a moving part.

Highly sensitive sensors have been developed which employ soft magneticmaterials, for example in the form of negative magnetostrictiveamorphous or nanocrystalline melt-spun wires and ribbons, and which arebased on the GMI effect.

When an ac drive current is passed through a magnetically soft (normallyamorphous or nanocrystalline) electrically resistive conductor, e.g. awire, ribbon or fibre, the ac voltage thereby developed is highlysensitive to the presence or application of an external magnetic field,particularly when the drive current frequency is greater than 100 kHz,the effect being known as the Giant Magneto-Impedance Effect (GMI). Thechange in voltage is understood as being a consequence of the dependenceof the skin depth of the conductor on the magnetic permeability.Interpretation of the GMI effect was introduced in 1994 simultaneouslyby Panina and Mohri Appl. Phys. Lett. 65 (1994) 1189 and Beach andBerkowitz Appl. Phys. Lett. 64 (1994) 3652.

Since the GMI effect can occur in long wires or ribbons, it is possibleto detect the integrated magnetic field along the path of the wire orribbon by using the appropriate hardware, as described in our copendingUK patent application GB 9814848.9 filed Jul. 9, 1998, and derivedInternational Patent Application WO 99/01967 and European PatentApplication 99926653.9. The external magnetic field to be measured maybe temporally invariant, but where it varies with time it is to beexpected that the ac impedance will show a corresponding variation.

The emphasis in the aforesaid patent application is the application of auniform bias field (see for example FIG. 6 of the application) to enableintegration of the external field to be measured along the length of aGMI material. By contrast, as will be described hereafter, in the sensorof the present invention a non-uniform bias field is deliberatelyapplied to the GMI material. The effect is dependent upon the componentof the field which lies along the length of the sensor, and it should beunderstood that the non-uniformity of the bias field mustcorrespondingly be a non-uniformity of amplitude as measured along thelongitudinal axis of the sensor.

Furthermore, because of the inverse magnetostriction effect in suchmaterials, the strong skin effect causes the impedance of the sensingelement (the electrically resistive conductor) to change with appliedstress S, this effect being termed the Giant Stress-Impedance effect(GSI). The physical mechanism of the impedance change is believed tosubstantially avoid cross-talk problems between orthogonal components ofthe stress tensor such as can arise with conventional strain gauges, forexample.

It has been found that the optimal drive frequency, i.e. the frequencyof the applied ac current, for GMI and GSI sensors lies in the MHzregion, which permits relatively easy integration into an rf (radiofrequency) communication system, for example for simple interfacing witha passive rf tag system. In turn this facilitates remote sensing ofstress or related factors in moving parts. Since it has also been foundthat such sensors have only a low power requirement for satisfactoryoperation, commonly as little as a few microwatts, it is possible tolocate a sensor element within a matrix, for example of plastics orelastic material, for remote interrogation with no external leads orother attachments.

The rf system may be any known system for sensing impedance changes inthe sensor element. For example, the wire may form part of a resonantcircuit which changes its resonant frequency as the impedance of thesensor element changes. Alternatively the sensor element could beincorporated into a balance bridge providing a frequency modulatedoutput rf signal.

Furthermore, the rf system could simply be a wire itself, which on itsown can be both as sensor and antenna, as in International PatentApplication No. PCT/SE00/00476 (Tyren et al) published under serialnumber WO 00/57147. In this sensor a temporally variant rf magneticfield (referred to as a magnetic sinewave bias field when the sensor ismagnetically driven), which interacts with the magnetic moments withinthe GMI/GSI material, is used both as a magnetic excitation for thesensor and also as a communication medium in that variations in thereturn excitation signal can be measured as an indication of the acimpedance of the wire, and hence stress in the wire. While the presentinvention will require some means of sensing the impedance variation ina GMI or GSI element, and while this could be effected by any of theforegoing means, it is primarily concerned with the provision of a biasfield which varies along the sensor. As will be explained later, such abias field makes regions of the sensor more or less sensitive accordingto position.

FIG. 1 is a schematic indication as to how the complex impedance Z(consisting of reactance X and impedance R) of a sensor wire elementconsisting of a 10 cm length of (Co_(0.94)Fe_(0.06))_(72.5) Si_(12.5)B₁₅(diameter of 125 μm) alters with the applied magnetic field (H) and itwill be seen that there is a very marked increase in impedance whenevera finite magnetic field is present. It will also be seen that theresponse is independent of direction of the field along the element,that the region of greatest sensitivity (rate of change of impedancewith field) is associated with the zero field position, and that thesensitivity falls as the magnetic field increases. Hereafter this regionwill be termed the “sensitive location”. However, precisely because suchsensors are so very sensitive to magnetic fields, including straymagnetic fields that commonly occur in stress measuring environments,applications thereof have heretofore been limited or impractical.

The latter point is illustrated in FIG. 2, which shows in schematic formthe characteristic variation in impedance of a GSI element in the formof a Co-based amorphous ribbon, 20 mm long, 1 mm wide and 20 μm thick,the exact composition being unknown, under various levels of appliedstress. While it will be appreciated that the impedance is markedlyaffected by the applied stress level, particularly at low levels ofapplied magnetic field, it will again be seen that variations in appliedmagnetic field also have a large influence on impedance, therebyrendering the measurement of stress by such a sensor unreliable. Itshould be noted that magnetic field refers throughout to the fieldcomponent parallel to the length of the wire. Effects of the fieldcomponent perpendicular to the wire tend to be negligible due to thelarge demagnetising effects in that direction.

The present invention provides a sensor comprising a generally elongatemagnetically soft amorphous or nanocrystalline electrically resistiveelement and biasing means for applying to the element a bias magneticfield, the component of said field directed along the length of thesensor having a spatially varying amplitude pattern along the element.Normally the sensor element will be of an amorphous or nanocrystallinemetal or alloy. The invention extends to a sensing device, a sensingarrangement, a method of reducing the sensitivity of the impedance of astress sensor element to magnetic fields, and a method of measuringstress in an object.

In one embodiment of the invention the bias field is arranged to reducethe effect of external ambient magnetic fields on the sensor response.In such a case, it is believed that the effect of applying the biasfield to the sensor element is to average out the GMI/GSI response toprovide a flat, or flatter, magnetic field insensitive response. Whilenot wishing to be bound by any theory, this is shown schematically inFIG. 3 where effective field H is plotted along the length L of thesensor for a bias field along the element having an amplitude whichvaries in a sawtooth manner along the element. In the absence of anyother magnetic field the sawtooth is symmetrically located about a zerofield line Ho as shown by dashed line, and sensitive locations a of thesensor occur each time the dashed line intercepts the zero field lineHo. In the presence of an additional ambient field directed along theelement the sawtooth is shifted as shown by the dotted line, and thesensitive locations are shifted along the sensor to locations b. Howeverthe number of sensitive locations remains the same, and accordingly theresponse of the sensor is insensitive to the presence of the ambientfield, relative to the case where the sawtooth bias is absent. Such anarrangement can be used to measure stress.

Care should be taken that the bias is not so strong as to make theimpedance response insensitive to stress as well as to stray magneticfields, and in this respect it has been found that it is possible tocontrol the stress sensitivity and magnetic field sensitivity bycontrolling the form and intensity of the bias magnetic field applied tothe sensor element. It will be understood that the optimum form of biasfield will depend on requirements for linearity of response, theexpected magnitude of stray fields in use for the application in hand,and on the manner in which impedance Z depends on magnetic field H andstress S.

In the foregoing arrangement, in which the sensitive locations of thesensor vary with external ambient field but the number of sensitiveregions remains constant, the exact position of the sensitive regionstends to be immaterial. In other embodiments, however, the bias field isarranged so as to control the location of a sensitive position orpositions of the sensor. This is shown schematically in FIG. 4 where theamplitude of the bias field 1 measured along the element is ramped alongthe sensor length L to provide a single sensitive location c. In theabsence of any external ambient field, the dc component of the biasfield can be altered to displace the sensitive location c, enablingstress to be measured locally at the location c. Alternatively, using asymmetrical bias field as shown, the location c is indicative of themagnitude of any external ambient field. This may be developed into amethod of detecting or mapping ambient fields, as will be explainedlater. Clearly the bias field pattern could be such as to provide two ormore sensitive locations simultaneously.

In one preferred embodiment of sensor the amplitude of the axialcomponent of the bias magnetic field (the “pattern”) varies periodicallyalong the element. One preferred variation is sinusoidal, butalternative patterns could be used as appropriate, including sawtooth(single or double ramp) and stepped patterns, or approximations thereto.

In zero external field a sinusoidal pattern puts relatively more of thesensor element into a high biased state (near A) than would a linearpattern (i.e. ramp, saw tooth or triangular configuration).

This means that when an external reverse field (approaching -A along theelement) is applied to a sinusoidally biased system, the total amount ofsensor element near a net-zero-field (high sensitivity state) is higherfor a sinusoidal bias field than when a linear bias field is used,resulting in a peak in the response to external field for a sinusoidalbias field, rather than the flatter response which would be obtained forthe linear bias field.

This is only true if the GMI response is small or relatively flat at netfields of around A, for more complex high field responses a more complexbias is required and preferred.

In another embodiment of the invention the bias field is not periodic.It may, for example, take the form of a linear or non-linear ramp.

The bias field (or at least the aforesaid pattern) may be predeterminedand time invariant. For example, the aforementioned sinusoidal fieldpattern may be applied by a magnetically loaded flexible mat, e.g. ofrubber, located adjacent the sensor element, and of approximately thesame length. The field pattern can be varied by the use of specificallydesigned magnetiser fixtures. In one embodiment, the mat was loaded witha SmCo-based magnetic filler, because its Curie temperature allowsoperation of the sensor under relatively high temperature conditions.Another embodiment utilises lower cost NdFeB-based magnetic filler whensuch high temperatures are not required.

Alternative ways of applying a fixed bias field include the use ofmagnetic or superconducting materials, magnetic coatings or cores, andsolenoid systems. Another way of applying the bias field is to makeintrinsic use of the magnetic properties of the sensing element. Forexample at the core of the amorphous or nanocrystalline wires there is adifferent domain configuration from the shell of the wire which couldprovide an intrinsic biasing field which varies in amplitude along theelement. An example of such a material would be Fe_(69.5)Cr₄Si_(7.5)B₁₅to which FIG. 5 relates.

However, it is also possible to apply a bias field where the pattern canbe varied. For example the pattern could be of a fixed functional form(e.g. sine wave, sawtooth, etc.) which is altered (for example swept) inamplitude (i.e. the pattern shape is retained but altered in magnitude—atemporal variation), and/or location along the sensor element (a spatialvariation). Alternatively or additionally the pattern itself could bechanged, i.e. a change in pattern other than merely by change inposition along the element. Such changes include variations in theactual shape and/or changes in the average level of the amplitude, i.e.the addition of a spatially constant bias offset—for example theaddition of a spatially constant bias to the field of FIG. 4 will enablethe sensitive location c to be moved along the sensor element.

Variations in the bias field pattern can be effected for example by theappropriate use of solenoids. Changes in bias pattern can be used tofacilitate the resolution of (magnetic field or) stress components alongthe sensor element. As mentioned above, when using a ramped biaspattern, the addition of a spatially constant bias pattern can producemovement of a sensitive location. A similar effect could be obtained bysweeping the existing pattern along the element, while altering theamplitude of the ramp, i.e. its slope, will control the length of thesensitive location. It will be understood that normally the rate of anychange in the bias field needs to be low relative to the rate ofmeasurement (c.f. Tyren above, where the applied field changes at a ratemuch greater than the rate of measurement).

Furthermore, by selectively coating regions of the sensor element withmore highly conductive coatings, these regions are effectively shortcircuited (there is only a surface current at the frequencies employed),and play a much reduced or negligible part in the sensor operation. Thismeans that only the stress S (or magnetic field H for GMI sensors)existing at uncoated regions will be included in the signal integration,thereby enabling the effect of certain undesirable local magneticfields, including components of biasing fields, to be masked out asrequired.

From the foregoing considerations it should be clear that by suitablyconstructing and/or controlling the sensor, it is possible to measurestress or magnetic field at one or more restricted locations along thelength of the sensor. This location or these locations can bepredetermined, for example by a design feature such as a patternedconductive coating, or controllable, for example by variation of theapplied bias magnetic field. Such a property is useful where it isdesired to avoid the difficulties and expense of soldering togetherseveral distinct elements or sensors.

The material of the sensing element may be a cobalt rich amorphousalloy, for example of Co_(72.5)Si_(12.5)B₁₅. Other alloys containingtraces of Mn, Fe, C, Nb, Ni, Cu, Mo and Cr can also be used. Othercompositions include Fe₈₁B_(13.5)Si_(3.5)C₂,Fe_(4.9)Co_(71.8)Nb_(0.8)Si_(7.5)B₁₅, Co₈₀B₂₀, Fe_(77.5)Si_(7.5)B₁₅,Ni₈₀Fe₂₀, Fe_(69.5)Cr₄Si_(7.5)B₁₅,(Co_(0.94)Fe_(0.06))_(72.5)Si_(12.5)B₁₅ and Fe_(73.5)Cu₁Si_(13.5)B₉.Cobalt rich amorphous or nanocrystalline alloys have extremely highmaximum tensile strength values typically of between 1 to 4 GPa, whichmeans that they are very suitable for use where a robust sensor isrequired. They also have a high elastic modulus typically of around 100GPa for a Co rich ribbon. In addition they exhibit high corrosionresistance.

Moreover, measurement on a CoSiB wire indicate that the sensor elementsare generally insensitive to changes in temperature at least in therange 20 to 150° C., making them suitable for use in environments wheresignificantly elevated temperatures are likely to be encountered. Itwill be appreciated that this is the case for measurements of stress inroad tyres, inter alia.

The sensor element may be in the form of a wire, ribbon or fibreproduced for example by melt spinning. Wire and ribbons are typically 10to 125 microns thick (minimum dimension). In manufacture, quenchingnormally results from the melt spinning process, and residual stressesarising therefrom couple with the magnetostriction to hinder domainrotation and so reduce the GMI/GSI effect. It is therefore preferred toanneal the quenched product to increase the sensitivity of the sensor,for example by furnace annealing, pulse current annealing or directcurrent annealing.

The sensor may comprise a single sensor element. It is possible to embedsuch a sensor comprising a sensor element, e.g. in the form of a ribbon,wire or large (elongate) fibre within an electrically relativelyinsulating supporting matrix, and in such a case the sensor will beresponsive to stresses applied to or transmitted through the matrix. Byway of example, coupling to the sensor element may be inductive,capacitive or via embedded conductors.

A typical example would comprise a ribbon or wire embedded in a vehicletyre for sensing stresses applied to the tyre when in use. It iscommonly recognised that the tyre to ground contact patch is the areawhere it is desirable to be able to instantly sense where the frictionalforce available is approaching the lower limit necessary for traction.Knowledge of the stress-strain dynamics of the tyre close to the ground,coupled with a model of the dynamic behaviour of the vehicle in responseto the contact patch forces would provide an almost instantaneousdetection of the dangers associated with changes in the nature of theroad surface, etc., such as incipient skids and aquaplaning, and mightalso provide information on tyre wear. A SAW sensor for such anapplication has been proposed in European Patent Application No.99114450.2. However, it is considered that the elastic properties of therubber/steel matrix will have a significant effect on the acoustic wavepropagation, and render such sensing difficult to employ in practice.

Alternatively, the sensor may comprise a plurality of sensor elements,e.g. in the form of discrete wires or ribbons, or as fibres. Where theelements are sufficiently large, they may be coupled together asdesired, for example two or more ribbons or wires in series to provide alarger sensor element. Such coupling may be by any suitable means suchas by direct electrical contact, or by coupling with non-magnetic wirestherebetween. Again the sensor elements may be embedded in anelectrically relatively insulating supporting matrix if desired

Where the sensor elements are relatively small, such as relatively shortfibres, it may be preferable to support them in an electricallyrelatively insulating supporting matrix. Where there is no directcontact between the elements, the properties of the sensor are thendetermined by the matrix as a whole, and the bias means may be arrangedto apply the bias field to the whole matrix. Preferably the individualelements have some degree of alignment along a preferred axis, but thematrix should still work as a sensor element even where the alignment issubstantially random. Ordering may be accomplished by any known means,for example by preferential orientation brought about by the process ofextruding the matrix material, or by the application of a magneticfield. Such fibres may have added benefits in terms of increasing themechanical strength of the supporting composite.

Where the sensor element is embedded in a matrix, or comprises a matrix,typical matrix materials therefor are plastics (synthetic resins) andrubbers. Commonly, these types of matrix material are electricallyinsulating.

Nevertheless, it is possible to employ matrix materials which have adegree of conductivity provided this is significantly less than that ofthe magnetic sensor elements(s) at the electrical frequency of use. Aninsulating matrix material may be rendered electrically conductive bysuitable loading with a conductive material, e.g. in fine particulateform. In such cases while the GSI effect will modify the impedance ofthe embedded fibre/wire/ribbon, this will then modify the impedance ofthe matrix as a whole, which will be sensed.

There are a number of examples of the use of the changing field at amagnetic sensor to enable displacement to be measured. An example isdisclosed in U.S. Pat. No. 4,119,911 (Johnson) in which a permalloysensor is used to measure field variations as magnets are moved todetect their motion. Permalloy is not normally a soft amorphous ornanocrystalline material, and does not show GMI or GSI effects. Thepresent invention relates to application of varying bias fields tochange the intrinsic properties of amorphous or nanocrystalline GMI andGSI materials.

Further features and advantages of the invention will become apparentupon consideration of the appended claims, to which the reader isreferred, and upon a reading of the following description of anexemplary embodiment of the invention made with reference to theaccompanying drawings, in which:

FIG. 1 shows in schematic form the characteristic variation in impedanceof a GSI element in the form of a wire of(Co_(0.94)Fe_(0.06))_(72.5)Si_(12.5)B₁₅ (10 cm in length and 125 μm indiameter.

FIG. 2 shows in schematic form the characteristic variation in impedanceof a GSI element in the form of a Co-based amorphous ribbon, 20 mm long,1 mm wide and 20 μm thick, the exact composition of which is unknown,under various levels of applied stress;

FIG. 3 illustrates in schematic form the desensitising effect of asawtooth bias field;

FIG. 4 illustrates in schematic form the use of a ramped bias field tocontrol the position where a sensor is sensitive;

FIG. 5 shows the impedance response for a 20 mm length of a ribbon madefrom the material Fe_(69.5)Cr₄Si_(7.5)B₁₅ with a width of 1 mm and athickness of 20 μm.

FIG. 6 schematically illustrates the field profile along the length ofone form of biasing element for use in the invention; and

FIG. 7 illustrates the results obtained according to one embodiment ofthe invention in the form of a graph of variation of impedance levelwith applied magnetic field for different applied stress levels.

FIG. 5 shows that the response is relatively field insensitive withoutan externally applied bias field. It also shows that for generally equalincrements of stress of around 71.2. MPa the differential response dropsprogressively, so that there is very little difference between responsesshown in plots D and E. The plots A0 and A1 represent measurements takenbefore and after the other measurements, respectively.

A rubber mat loaded with NdFeB magnetic powder was rolled into amulti-turn cylinder and subjected to a diametric uniform magnetic fieldpulse from a Hirst Magnetiser system to produce when the mat M isstraightened a magnetic intensity distribution (field profile) along themat length L (width W). NdFeB is neodymium iron boron, a permanentmagnetic material similar to SmCo. The resulting field profile F isschematically illustrated by the arrows in FIG. 6 for a two-turncylinder, (i.e. 4 pole pitches, along a 25 mm long mat). The mat lengthwill in general be chosen to substantially match the length of theresistive element with which it is to be used, which can be anythingfrom a few microns to several metres, permitting either point orintegral measurement of stress.

The straightened NdFeB loaded mat is placed adjacent to a 20 mm longFeCoSiB amorphous ribbon to produce a sensor according to the invention,and FIG. 7 shows in schematic form the impedance thereof at variousstress levels as a function of an externally applied magnetic field.Each horizontal plot relates to one stress level. It will be seen thatthe presence of the biasing mat leads to a characteristic which varieswith applied stress, but which is substantially independent of the levelof the externally applied magnetic field.

While particular reference has been made to the measurement of stress ina vehicle tyre, it will be appreciated that there are other applicationsof the sensor of this invention, including the measurement of stress inaircraft wings and machine parts, including moving parts, and in themonitoring of stress levels in structures where stray magnetic fieldsconstitute a problem, such as bridges where fields may be produced bymoving vehicles. A pressure transducer, e.g. for use within a tyre, maycomprise such a sensor.

1. A sensor comprising a generally elongate magnetically soft amorphousor nanocrystalline electrically resistive element adapted to exhibit adetectable change of property when exposed to a stress or magnetic filedto be sensed; a detector for detecting such change of property andbiasing means for applying to the element a bias magnetic field, thecomponent of said field directed along the length of the sensor having aspatially substantially varying pattern of amplitude along the element.2. A sensor according to claim 1 wherein said pattern is predetermined.3. A sensor according to claim 1 wherein the biasing means iscontrollable to effect change in said pattern.
 4. A sensor according toclaim 3 wherein said change is in the amplitude of the pattern.
 5. Asensor according to claim 3 wherein said change is in the position ofsaid pattern relative to the length of the element.
 6. A sensoraccording to claim 3 wherein said change is in the pattern itself.
 7. Asensor according to claim 1 wherein said amplitude variation isperiodic.
 8. A sensor according to claim 1 wherein said pattern includessubstantially linear ramps, or approximations thereto.
 9. A sensoraccording to claim 1 wherein said element has portions along its lengthcoated with a highly conducting material.
 10. A sensor according toclaim 1 wherein said element is in the form of a wire, ribbon or fibre.11. A sensor according to claim 1 wherein said element is melt spun. 12.A sensor according to claim 11 wherein said element is annealed afterbeing melt spun.
 13. A sensor according to claim 1 wherein said elementexhibits a small negative magnetostriction.
 14. A sensor according toclaim 1 wherein said element exhibits a small positive magnetostriction.15. A sensor according to claim 1 wherein the element is formed of acobalt rich alloy.
 16. A sensor according to claim 1 wherein the elementis embedded in a matrix.
 17. A sensor according to claim 1 wherein aplurality of said elements are embedded in a matrix.
 18. A sensoraccording to claim 16 wherein said matrix is electrically insulating.19. A sensing device comprising a sensor according to claim 1 andcurrent supply means for supplying an alternating current to theelement.
 20. A device according to claim 19 wherein the alternatingcurrent frequency lies in the radio-frequency range.
 21. A deviceaccording to claim 20 wherein the alternating current frequency is atleast 100 kHz.
 22. A device according to claim 21 wherein thealternating current frequency is at least 1 MHz.
 23. A sensing deviceaccording to claim 19 wherein said current supply means is directlycoupled to the sensor element.
 24. A sensing device according to claim19 wherein the current means is directly coupled to the element byinductive or capacitative or rf coupling.
 25. A sensing arrangementcomprising a device according to claim 19 and measuring means formeasuring the alternating voltage generated in the element by thealternating current.
 26. An arrangement according to claim 25 whereinthe element forms part of a resonant circuit.
 27. An arrangementaccording to claim 25 wherein the element forms part of a bridgecircuit.
 28. A method of reducing the sensitivity of the impedance of agenerally elongate magnetically soft amorphous or nanocrystalline stresssensor element to external magnetic fields, the method comprisingapplying along the sensor element a bias magnetic field, the componentof said field directed along the length of the sensor having a spatiallysubstantially varying amplitude pattern along the element.
 29. A methodof measuring stress in an object by securing thereto a stress sensorcomprising a generally elongate magnetically soft amorphous ornanocrystalline stress sensor element, supplying an ac current to theelement, and measuring the impedance of the element, wherein a biasmagnetic field is applied with an amplitude which varies with positionalong the element, the component of said field directed along the lengthof the sensor having a spatially substantially varying amplitude patternalong the element to reduce its sensitivity to external magnetic fields.30. A method according to claim 28 wherein said pattern ispredetermined.
 31. A method according to claim 28 wherein said patternis changed.
 32. A method according to claim 28 wherein the magnitude ofsaid pattern is changed.
 33. A method according to claim 31 wherein theposition of said pattern along the element is changed.
 34. A methodaccording to claim 29, wherein the object is a type, or part of apressure transducer, or a part of a bridge, or a part of an aircraft, oran aircraft wing.
 35. A tyre, or a pressure transducer within a tyre,having embedded therein a sensor according to claim
 1. 36. A vehicleprovide with a tyre according to claim 35 and having detecting means fordetecting changes in the impedance of the sensor element.
 37. A vehicleaccording to claim 36 wherein the detecting means is secured to a fixedpart of the vehicle.
 38. An aircraft having secured thereto a stresssensor according to claim
 1. 39. An aircraft according to claim 38wherein the sensor is secured to a wing.
 40. A bridge having securedthereto a stress sensor according to claim
 1. 41. A method of sensingstress applied to a flexible matrix material comprising embedding asensor according to claim 1 in the matrix and measuring its impedance.42. A method according to claim 41 wherein the matrix material iscarcase of a tyre.